Dental procedures and apparatus using ultraviolet radiation

ABSTRACT

An improved dental procedure and apparatus where ultraviolet radiation pulses are used to etch selectively both hard tissue and soft tissue in dental procedures. There exists distinct ablation thresholds for hard and soft tissue which are dependent on the material being abated for a given wavelength of the ultraviolet radiation. Sufficient differences in ablation threshold exist for enamel, dentin, and carious material, thereby allowing dentists to perform both hard tissue and soft tissue procedures without excess damage to healthy enamel, dentin or other pulp structures.

This is a division of application Ser. No. 08/271,385, filed Jul. 6,1994, now U.S. Pat. No. 5,435,724 which is a File Wrapper Continuationof Ser. No. 08/026,333, filed Mar. 4, 1993 (now abandoned).

DESCRIPTION

1. Field of the Invention

This invention relates to improved procedures and apparatus forprocedures on teeth using ultraviolet radiation, wherein ultravioletlaser pulses having selected energy fluences can be used to performseveral different procedures on different materials found in teeth.

2. Background Art

Lasers are optical devices which produce intense and narrow beams oflight at particular wavelengths by stimulating the atoms or molecules ina lasing material. Many types of lasing materials are known includinggases, liquids and solids. The lasers are typically named in accordancewith the element or compound that emits light when energized, such ascarbon dioxide, argon, copper vapor, neodymium-dopedyttrium-aluminum-garnet (Nd:YAG), erbium, holmium, rare gas halide gasmixtures such as ArF, XeCl, KrF (excimers) etc., alexandrite, ruby,Ti:sapphire and many dyes. When applied to human tissue, the beam oflight produced by the laser will be partially absorbed in a processwhich typically converts the light to heat. This is used to change thestate of the tissue for purposes of etching or cutting. In the case ofnonultraviolet radiation-producing lasers, the dominant mechanism forcutting or etching is a thermal one. However, in the case of ultravioletlaser pulses having energy fluences in excess of a threshold dependentupon the wavelength of the radiation and the material being irradiated,a "cool" etching is achieved in which there is minimal heat transferredto the surrounding tissues. Instead, the energy of the ultravioletradiation pulses is primarily transformed into the kinetic energy of theparticles which explode or ablate from the tissue being irradiated. Thisfundamental discovery and its application for both medical and dentalpurposes is described by S. E. Blum et al in U.S. Pat. No. 4,784,135.

In the development of a laser system for specific dental and medicalapplications, factors to be considered are the wavelength of the lightproduced by the laser, the pulse width of the radiation pulses, theenergy per pulse, the laser beam spot size on the target and theapparatus and method of delivery of the laser light to the tissue to beirradiated. It is necessary to deliver a precise amount of light to thetissue, whether the mechanism for cutting is a thermal on or ablativephotodecomposition as can be achieved through the use of pulsedultraviolet radiation. If there is application of energy of highintensity to the tissue, rapid absorption and heating can occur whichcan cause undue damage to areas surrounding the irradiated region. Ingeneral, pulse-type operation is preferred rather than continuous waveirradiation, since delivery of a series of pulses provides an additionalcontrol over the interaction, absorption of radiation, and the overallprocess. This leads to more control of the etch depth and the degree ofdamage to surrounding tissue.

Medical research on the use of lasers has been ongoing for many years.The use of lasers in the field of ophthalmology to correct, for example,myopia, is the subject of considerable research effort where goodsuccess in noted. Many different types of lasers have been used, withthe more significant results now being produced through the use of raregas halide excimer lasers providing pulsed ultraviolet radiation. Inaddition to this application, lasers are now used in a variety ofmedical applications such as gynecology, urology, dermatology,angioplasty, and plastic surgery. Lasers are also used in generalsurgery in connection with surgical procedures concerning the ear, nose,and throat as well as in the treatment of gastrointestinal ailments. Thegenerally listed advantages of lasers for some medical applicationsinclude reduced surgical pain, reduced infection and bleeding, reducedscarring, and less post-operative pain, as well as a reduced need forpost-operative analgesics.

The use of lasers in dentistry has also been the subject of considerableresearch and development activity. These prior efforts have involved thecontrolled application of laser thermal effects to soft or hard tissue.Problems to be avoided in such laser dentistry include the destructionof teeth by heat and often unsatisfactory techniques for delivering thelaser pulses into confined regions in the mouth. Various types ofarticulated arms and fiber optic delivery systems have been developedfor these latter purposes. At this time, testing of various types oflaser systems in laser dentistry is occurring, where the commonly usedlasers are the Nd:YAG laser, CO₂ lasers, holmium lasers, argon lasersand erbium lasers.

Dentistry involves soft tissue procedures as well as hard tissueprocedures. The soft tissue procedures include the removal of excess ordiseased gum tissue, contouring of gums, performing biopsies, preparinggums for crown and bridge impressions, trimming the gums to fit thecrowns and bridges, treating various types of gum disease and infectedpockets between the gum and teeth, and hemostatis (control of bleeding).Hard tissue procedures include drilling, removal of decay from teeth,preparation of teeth for filling, increasing of hardness of dentin torender teeth less susceptible to decay, removing stains, anddesensitizing and anesthetizing teeth. U.S. Pat. No. 5,055,048 describesa Nd:YAG dental laser assembly useful for many different dentalprocedures as outlined therein.

The pulsed Nd:YAG laser has dominated the dental market, but use of thislaser is limited to soft tissue treatments, such as removing and shapinggums. This laser provides a wavelength of 1.06 microns which is onlyslightly absorbed by water. However, the Nd:YAG laser cannot be usedeffectively to remove hard tissue and is often not as desirable whenprecise control of heating adjacent tissue is necessary (as for examplewhen a small piece of gum is to be removed without harming an adjacenttooth). A CO₂ laser is more appropriate for soft tissue dentalapplications, but it is not suited for hard tissue use because theenergy level needed is very high and causes damage to nearby tissue. Inthe field of cosmetic and restorative dentistry, an argon laseroperating at about 488 nm appears to be preferred. This type of lasercan be used to polymerize sealants in pits and fissures and can be usedto quickly cure restorative materials.

The erbium laser may be more suitable as a hard tissue dental tool (ifused with water) since it may be versatile and safe to use. Anotherpossible candidate for hard tissue applications is a short-pulse,high-energy CO₂ laser. However, it is not clear that these type oflasers can be used for all dental applications, it being apparent thattheir use will be limited to selected dental procedures.

From the foregoing, it is apparent that the development of laserdentistry is in its early stages and that all of the commerciallyavailable lasers have inherent disadvantages in terms of their limitedapplicability to selected dental procedures. A major problem encounteredwith these lasers is that all of them rely on the absorption of laserenergy to produce heat, which in turn is used for tissue removal. Thiscreates problems dependent on the type of material irradiated, as thethermal diffusion and thermal mechanism varies with different materialsand is more difficult to control. Because there is heat spreading toregions surrounding the irradiated area, destruction of adjacent tissueis likely to occur. Additionally, the energies used to provide tissueremoval are often such that the applications of the laser pulses must bevery strictly controlled in area. In many circumstances, it has beenfound that laser dentistry in its present state does not affordsignificant advantages over conventionally used instruments such asmechanical drills. Of particular significance is that it is notpresently possible to perform drilling operations with most lasers, asnone of the commercially available lasers can be used to cut enamel.Still further, none of the commercial lasers works very well to removedental carries and to provide possibilities for root canal surgery.

In addition to the visible and infrared lasers that have been used fordental procedures, U.S. Pat. No. 4,784,135 described the use ofultraviolet lasers, such as excimer lasers, for dental work. Generally,pulsed UV laser are used where the energy fluence per pulse issufficient to produce ablation. A follow-up article by J. Wynne and R.Lane (Lasers and Applications, p. 59, November 1984) describes ablationof enamel and dentin by UV laser pulses, but does not address removal ofcaries, critical ablation thresholds or techniques for practicaldentistry.

In addition to the foregoing patent, U.S. Pat. No. 5,107,516 describes atwo-laser feedback system that employs ablation to remove arterialplaque and mentions possible dental applications. German patent DE 4015066 A1 describes a technique in which differential reflectometry is usedto determine the duration and/or energy of each laser pulse, where thelaser can be a UV laser used for removal of dental caries. AnotherGerman patent DE 3800555 A1, based on PCT application PCT/DE89/00010,describes the use of an ARF excimer laser delivery system at 193 nm toablate hard dental material such as dentin or enamel. A delivery systemincluding a sealed and evacuated articulated arm and reflectors isemployed to delivery the UV radiation.

In these prior art systems, UV radiation is used for dental applicationsbut additional means are usually required to ensure safe operation. Thisis exemplified by DE 4015066 where differential reflectometry is used ina feedback loop to provide laser control so as not to remove healthytissue. In contrast with this the present invention does not requirereflectance or spectroscopic techniques to identify target tissue.Instead, material removal is controlled by utilizing newly discovereddiffering ablation thresholds for different types of dental material.Based on the discovery and recognition of these different thresholds,various windows of operation are defined which allow a single laser tobe used for several different procedures without risk to the patient andwithout the need for sophisticated target tissue identification.

Accordingly, it is a primary object of this invention to provideimproved laser dentistry in which a single laser system can be safelyused to do various dental procedures including both hard and soft tissueprocedures.

It is another object of this invention to provide improved laserdentistry where the energy fluence per pulse can be changed to allow alaser to do both hard and soft tissue removal.

It is another object of this invention to provide laser dentistry whichyields minimum cellular destruction to tissue at the margins of theirradiated volume.

It is another object of this invention to provide improved laserdentistry which does not rely on thermal mechanisms as the primarymechanism for hard and soft tissue removal.

It is another object of this invention to provide an improvedultraviolet laser system for dental applications in which the risk ofcontaminiation is reduced during the procedure by the use of ultravioletradiation providing germicidal sterilization.

It is a further object of this invention to provide laser pulse removalof hard tissue in a safe and effective manner.

It is another object of this invention to provide an ultraviolet dentalprocedure and apparatus in which ablative photodecomposition is used toprovide windows of operation wherein the same laser can be used for bothhard and soft tissue applications and wherein automatic control of thelaser output pulses is obtained.

It is another object of this invention to provide an improved lasertechnique for fluoride treatment of teeth.

BRIEF SUMMARY OF THE INVENTION

A technique and apparatus for improved laser dentistry are described inwhich pulsed ultraviolet light, preferably from a laser, is used toselectively remove tooth material. By directing the laser beam onto thesurface of the tooth at a location where material is to be removed,carious lesions, dentin, and enamel can be removed to a controllabledepth by using the correct combination of laser fluence and number ofpulses, with minimum damage and heat being produced in the tooth at themargins of the excised material. Ultraviolet light, at an energy fluenceabove the threshold for removing tooth material, is absorbed in a thinlayer of irradiated material and is delivered in a time that is shortcompared to the time for the absorbed energy to thermally diffuse intoadjacent volumes. In the practice of this invention, pulsed ultravioletlight can be used to ablate carious material, dentin, or enamel, eachwith a defined energy fluence threshold below which the material is notremoved. This provides windows of operation, or energy regions oftolerance, enabling a dentist to safely use an ultraviolet laser inseveral procedures. The depth of material removed per pulse increasesfor increasing fluence above the threshold. Unexpectedly, the thresholdfor removal of carious material by UV light was greater than that fordentin. However, for a given fluence above threshold, the amount ofcarious material removed is much greater than the amount of dentin thatis removed.

An apparatus suitable for the delivery of laser radiation havingselectable energy fluence can be provided by an optical system using aseries of mirrors and lenses to focus the laser beam onto the tooth tobe irradiated. As an alterative, the laser energy can be delivered by anoptical fiber delivery system having suitable optics (lenses, etc.) atthe end of the fiber to direct the laser beam into the fiber and tofocus the light emerging from the fiber onto the tooth. A beamhomogenizer may be used to ensure that the beam is uniform in intensityto avoid the presence of hot spots. This can be done by a series oflenses or by a series of highly transmitting channels that cause thevarious parts of the beam to crisscross and overlap. If an optical fiberis used, total internal reflections will accomplish this result. Meansare provided to allow the dentist to manipulate the tool to direct thelaser beam to the desired place on the tooth.

The pulsed ultraviolet radiation can be provided by any source thatproduces radiation having energy fluences sufficient to meet thethreshold for removal of the particular material of the tooth to beirradiated. Excimer lasers are available for providing variousultraviolet wavelengths, for example at 193 nm, 248 nm, 308 nm, and 351nm. Solid stat lasers such as frequency-multiplied near infra-red orvisible lasers such as Nd:YAG and Ti:sapphire, diode lasers andmicrolaser or microlaser arrays can also be used. U.S. Pat. No.5,144,630 describes several solid state lasers which can producecoherent radiation at multiple wavelengths in the ultraviolet range andinfrared range. Since DNA has an absorption peak at about 250 nm, it maybe preferable to avoid lasers providing an ultraviolet output near thiswavelength. Particularly suitable wavelength ranges appear to be about185-220 nm and 300-400 nm.

Automatic feedback control systems based on the unique ablationcharacteristics of the specific material (enamel, dentin and carries)being ablated are described. These systems can also be used to alert thedentist about the presence of harmful biological contaminants, and tocontrol the UV laser pulse repetition rate to ensure patient comfort.

These and other objects, features, and advantages will be apparent fromthe following more particular description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an apparatus for applying suitablepulsed ultraviolet radiation on a tooth, using a series of reflectorsand lenses together with a beam homogenizer to provide pulsed laserradiation of proper wavelength and energy fluence.

FIG. 2 is a schematic illustration of another apparatus for performinglaser dentistry in accordance with the present invention, where anoptical fiber delivery system is utilized.

FIG. 3 is a plot of energy fluence versus etch depth per pulse for theablative photodecomposition of dentin, using pulsed ultravioletradiation at 308 nm.

FIG. 4 is a plot of energy fluence versus etch depth per pulse for theablative photodecomposition of enamel, using an excimer laser providingan output radiation at 308 nm.

FIG. 5 is a plot of energy fluence versus etch depth per pulse for theremoval of carious material using ultraviolet light pulses at 308 nm.

FIG. 6 is a combined plot of energy fluence versus etch depth per pulsefor the removal of enamel, dentin and carious material using ultravioletradiation pulses at 308 nm. The data from FIGS. 3-5 are plotted on acommon scale to facilitate comparison of ablative photodecomposition ofenamel, dentin and carious material.

FIGS. 7 and 8 schematically illustrate a dental tool in accordance withthe present invention where a suction tube is used to removeparticulates and other matter in the plume of ablated material from atooth.

FIG. 9 is a schematic illustration of a laser dental tool which uses thesignature of the type of tooth material being ablated to automaticallyadjust the characteristics of the UV laser ablation pulses.

FIG. 10 is a schematic illustration of a laser dental tool which can beused to protect the dentist from contamination by adverse biologicalproducts produced in a patient's mouth during UV laser treatment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

When human teeth are exposed to pulsed ultraviolet light from a laserabove a threshold energy fluence, material will be ablated from thesurface of the tooth. Below the threshold energy fluence, no material isremoved. It has been discovered that healthy enamel, healthy dentin, andcarious lesions each have different energy fluence threshold forablation, as well as different absorption coefficients which describedthe characteristic depths to which UV radiation is absorbed in thedifferent materials. This allows the use of a single UV laser system toaccomplish several dental procedures safely, in which different windowsof operation can be defined.

FIGS. 1 and 2 illustrate suitable forms of an apparatus that can be usedto deliver ultraviolet radiation to teeth.

In FIG. 1, a pulsed ultraviolet laser 10 provides a light beam 12 in thewavelength range less than about 400 nm. Rare gas halide excimer laserscan be used as the radiation source for providing ultraviolet outputs at193 nm (ArF), 248 nm (KrF), 308 nm (XeCl), and 351 nm (XeF).Additionally, a solid state laser such as a frequency-tripled Nd:YAG canbe used to provide an output at 355 nm. In order to be able to block thelaser beam 12 on demand, a shutter 13 is provided. A beam homogenizer 14is optionally provided to ensure that the beam is uniform in intensity.Beam homogenizers are known in the art, as can be seen by referring toY. Ozaki and K. Takamoto, Applied Optics, Vol. 28, p. 106 (1989). In aparticular embodiment, homogenizer 14 can be comprised of a series oflenses (Y. Ozaki et al., ibid) or a series of highly transmittingchannels (M. Wagner) et al., Measurement Science and Technology, Vol. 1,p. 1193 (1990)) that cause the various parts of the beam to crisscrossand overlap, thereby smoothing out intensity variations and eliminatinghot spots. Since excimer lasers are multi mode lasers, hot spots mayoccur. The homogenizer generally breaks the beam into small beamletswhich are then recombined to provide a more uniform intensity across thebeam cross section.

Beam 12 is then directed to a rotatable mirror 16 from which it isreflected to a lens 18. The lens is chosen to have a focal lengthsufficient to cause a focused point of light to be delivered to thetooth 20, and particularly to a localized area 22 to be irradiated. Area22 can be, for example, an area of a lesion, such as a carious lesion onthe tooth. In FIG. 1, the teeth are schematically illustrated as is asection of the person's gum 24. In practice, the apparatus (evenincluding the laser 10) can be located in an articulated arm of the typecommonly used by dentists.

As an alternative, the rotatable mirror 16 and lens 18 can be replacedby a curved concave mirror. This will deflect the light beam and alsofocus it. A mirror with a coating of a highly reflective material, suchas Al or a multilayer dielectric, can be used.

FIG. 2 shows another embodiment for an ultraviolet delivery system to atooth to be irradiated. Components having the same function as thoseshown in FIG. 1 are given the same reference numeral. Thus, the laser 10in FIG. 2 provides output laser pulses 12 of wavelength less than about400 nm. Shutter 13 is used to block the light beam 12 as desired by thedentist. A lens 26 is used to direct and couple the focused laser beam28 into an optical fiber 30, which carries the laser pulse. The laserpulses exit fiber 30 to provide a pulse train 32 which impinges upontooth 20, and particularly on the area 22. Due to the multiple totalinternal reflections which occur in fiber 30, beam homogenization willautomatically occur to ensure that the output beam 32 is sufficientlyuniform in intensity over its cross section.

The apparatus of FIG. 2 is desirable since the dentist can hold inhis/her hand a tool consisting of the delivery end of the optical fiberdelivery system. The dentist can then manipulate this tool to direct thelaser beam to the desired place (22) on the tooth. A very short focallength lens 34 located at the delivery end of optical fiber 30 enablesthe dentist to hold the tool close to the tooth and to provide a veryfocused beam at the area 22 to be irradiated. The end of the fiber canbe shaped to provide a lens, or a lens can be attached to the deliveryend of the fiber. Thus, material will be ablated from the tooth in avery controlled area easily observed by the dentist.

EXAMPLES

Quantitative experiments were performed on human dentin, enamel andcarious lesions. Approximately 2 mm thick cross sections were cut fromthe middle third of the crowns of several molar teeth using a BuchlerIsomet saw with a diamond wafering blade. Both surfaces of each sectionwere polished with 320 followed by 600 grit wet carbimet polishingpaper. Sections were then immersed in 6% citric acid and shaken for 2minutes to remove the smear layer. They were then rinsed with deionizedwater and stored in 70% ethanol. Carefully selected areas of thesecross-sectioned human teeth were exposed to a given number of pulses oflight from a 308 nm XeCl excimer laser at a given fluence. Using amechanical profilometer, the depths of the resulting ablation trencheswere measured. The beam energy fluences were then plotted on semi-logpaper as a function of ablation trench depth per pulse, and a straightline was fitted to the data. Assuming Beer's law correctly describes theabsorption of the UV radiation within the tooth, the followingexpression will describe the relationship between the amount of materialremoved and the applied energy fluence:

    F=F.sub.o e.sup.-αl,

where

where F is the fluence of the laser beam at a depth l into theirradiated tooth,

F_(o) is the fluence of the UV radiation at the surface of the regionbeing irradiated, and

α is the absorption coefficient for the dental material being ablated.

By measuring the depth d per pulse of an ablated hole as a function oflaser fluence, the fluence threshold for ablation F_(th) and thecoefficient α for the ablated material can be determined:

    F.sub.th =F.sub.o l.sup.-ad

The energy per unit E being deposited at a depth l in the irradiatedmaterial is the product of the energy fluence F at that depth and theslope α of the logarithm of the applied fluence versus depth plot. Thus

    E=Fα

When the energy per unit volume E exceeds the threshold energy per unitvolume E_(th), ablation will result. Since the slope α is a constant foreach dental material (enamel, dentin and carious material),

    E.sub.th =F.sub.th α

Plots of data for the ablation of dental materials are shown in FIGS.3-6. FIG. 3 illustrates the data for dentin ablation, FIG. 4 illustratesthe data for enamel ablation, and FIG. 5 illustrates the data forablation of carious material. FIG. 6 is a plot which combines the datain FIGS. 3-5.

Referring to FIG. 3, for dentin, the ablation threshold fluence, F_(th),is 1.04±0.06 J/cm² and the absorption coefficient α is 2.7±0.1 μm⁻¹. Theenergy threshold E_(th) is thus 2.8×10⁴ J/cm³.

Referring to FIG. 4, for enamel F_(th) is 5.9±0.3 J/cm², α is 3.8±1.5μm⁻¹, and E_(th) is 22×10⁴ J/cm³.

The ablation depth per pulse for dentin is approximately 0.3 μm for anincident fluence of 2.3 J/cm², at which fluence enamel would not beablated. For enamel, the ablated depth per pulse is approximately 0.03μm for an incident fluence of 6.6 J/cm². From these measurements, it isapparent that there is a wide window of fluence where dentin can beablated without removing or damaging enamel that is unavoidably exposedto the laser beam.

Experiments were also performed to determined the threshold energyfluence for the ablation of carious material. These studies wereperformed at 308 nm using an excimer laser. A tooth having cariestherein was preserved with an alcohol solution and then a flat surfacewas made by polishing with emery paper. This facilitated visualinspection before and after irradiation to determine the etching effectof the ultraviolet laser pulses. Five holes of about 0.3 mm²cross-sectional area were then etched into the carious material byapplication of UV laser pulses of different fluence. The depth per holewas measured using a mechanical profilometer profilometer, and theresulting data are plotted in FIG. 5. As noted in FIG. 5, F_(th) forcarious material was determined to be 1.36 J/cm², while the slope (α) ofthe curve is 0.25 μm⁻¹. E_(th) for carious material is thus 0.34×10⁴J/cm³.

The foregoing experiment was then repeated with the same tooth, afterrinsing with an alcohol solution and polishing a different region of thetooth. Five new holes were etched into the carious material. The samemask was used as was used in the first experiment, the holes being about0.3 mm², in cross-sectional area. Again, the depth of each hole wasmeasured and plotted against the known energy fluences that wereapplied. The wavelength of the incident radiation is again 308 nm. Theresults were essentially identical to those of FIG. 5.

It was initially surprising that the energy threshold for ablation ofdentin is less than that for carious material. Since carious material isprimarily organic in nature, it was expected that the ablation thresholdfor the carious material would be less than that for the harder dentinmaterial. However, as noted above, it is the threshold energy per unitvolume which must be exceeded in order to have ablation. Comparing theslope of curve in FIG. 5 with that in FIG. 3 indicates that the curve inFIG. 5 is much flatter than the curve in FIG. 3 for dentin ablation.Thus, the absorption coefficient α for carious material is less thanthat for dentin, the energy threshold is more than 8 times smaller andmuch more carious material will be removed than healthy dentin for thesame fluence of applied ultraviolet energy. The amount of cariousmaterial removed at a given energy fluence can be as high as ten timesthat of the amount of dentin removed. Since the light penetrates deeperinto carious material, a higher threshold fluence is required to produceablation for caries, in contrast with the ablation threshold for dentin,where the light penetrates less. When operating above the thresholdenergy for carious material, dentin will be ablated as well as cariousmaterial, but the amount of dentin that is ablated will be significantlyless than that of the carious material. The data from FIGS. 3-5 areplotted in FIG. 6 on a common scale, to facilitate comparison. FIG. 6illustrates the different threshold and rates of removal of thesedifferent types of tooth material.

In order to provide a safety factor, i.e., not to excessively ablatehealthy dentin when it is desired to remove carious material, a"signature" is required to indicate to the dentist the nature of thematerial being ablated. Above the threshold fluence for caries, a loudpopping sound will be heard, accompanied by an orange-colored plume.When all of the carious material has been removed and health dentin isexposed to the ultraviolet pulses, the popping sound will become softerbecause the amount of material being ablated is less. This provides anindication of the material being ablated.

One way to view the ablation process is that sufficient energy per unitvolume must be deposited in the material to turn it into a gas. Thematerial will expand and blow out of the etched hole. The energy fluencemust be sufficiently high that the energy per unit volume will causethis to occur. Whether the mechanism is characterized as thermal bondbreaking or photochemical bond breaking is not germane to the successfulremoval of all types of dental tissue in accordance with this invention.The term ablative photodecomposition, or ablation, is used in a genericsense to include both thermal bond breaking and photochemical bondbreaking where the application of ultraviolet radiation occurs in amanner to blow away irradiated material at a rate sufficient that thereis minimal thermal diffusion into the nonirradiated material. It isrecognized that, if the repetition rate of ultraviolet pulses is toohigh or if the pulse width is too great, the ablation products will notbe able to "blow-off" from the irradiated material fast enough toprevent excess thermal diffusion. In this case, thermal damage of theedges of the irradiated region could result.

A signature of ablation is an easily discernible popping sound that issynchronous with the laser pulses. This sound is heard only whenmaterial is ablated, as confirmed by post-ablation measurements. Thesound is generated by gaseous material ablating off the surface.Accompanying this sound is an orange-colored "jet" emanating from theablated surface. These two signatures become more pronounced withincreasing fluence above threshold. They are absent below the ablationthreshold. Consequently, this sound and the orange-colored jet present asimple and immediate way to determine what sort of material is beingablated by the laser pulses. For a given fluence above the threshold forablating enamel, the popping sound and jet size are much more pronouncedwhen this fluence is directed onto dentin than onto enamel. Therefore,when ablating enamel at the surface of a tooth, the popping sound andorange-colored jet will strengthen dramatically as soon as the ablationtrench has penetrated through the enamel to the underlying dentin. Thisprovides as in-situ indicator to reduce or shut off the laser fluence toprevent unwanted penetration into the dentin. Of course, the dentist canalso stop to observe the results of the UV irradiation at any timeduring procedure.

Decayed material, which is predominantly organic, has an ablationthreshold below that of enamel. Decayed material was selectively removedfrom underlying dentin and enamel using an energy fluence of 2.3 J/cm²(308 nm), well below the threshold fluence for ablating enamel.Additionally, an energy fluence of 2.3 J/cm² was able to ablate decayedmaterial more effectively (i.e., at a significantly greater rate) thandentin, as indicated by the more pronounced popping sound and orange jetsignatures.

It has been found that organic materials in dentin tubules can beremoved by UV laser radiation at thresholds less than the ablationthreshold for dentin. The organic material in the tubule is removedwithout the necessity for clogging the tubules. This selective techniquemay therefore assist in desensitizing teeth.

As noted, other UV wavelengths can be used. The selection of 308 nm forthe experiments illustrated in FIGS. 3, 4 and 5 was based on theexistence of optical fiber delivery system for 308 nm radiation, theseoptical fibers being capable of transmitting sufficient energy fluencefor material ablation. Examples of materials suitable for optical fibersat this wavelength are the following: quartz, fused silica, and selectedsapphires.

Lens material suitable for application with ultraviolet wavelengthsinclude those fabricated of quartz, calcium fluoride, magnesiumfluoride, fused silica, and UV sapphire.

This laser system has also been shown to remove stain from teeth. Inalmost all cases, staining is a discoloration of the organic pellicleoverlying the enamel. This material is organic in nature and consists ofsalivary proteins which form a deposit of only a few microns thickness.Instead of removing stain in the traditional way using an abrasivetechnique, ultraviolet pulses can be used. The organic stained materialhas an ablation threshold below that of normal enamel and can be easilyremoved without affecting the underlying enamel. Tartar, or calcifiedplaque, can also be removed by UV radiation.

It may also be possible to use ultraviolet laser radiation to providedentin desensitization for those people who have very sensitive teeth.This desensitization would be achieved by creating a sufficient increasein temperature to cause the dentin tubules to be sealed.

As reported by B. D. Goodman and H. W. Kaufman in the J. DentalResearch, page 1201, October 1977, laser radiation can be used toenhance fluoride uptake into human tooth enamel and also reduces enameldissolution in acid and therefore its susceptibility to tooth decay. Inthis prior work, an argon laser was used at a wavelength of 514.5 nm. Ina later work, a CO₂ laser was shown to be even more effective (J. of theJapanese Society of Laser Medicine, vol. 6, pp. 231-234, 1986). In thepractice of the present invention, an ultraviolet laser source can beused in combination with a fluoride carrier in which the fluoride (NaFor another fluoride) is dissolved in a solution that does not char uponlaser light exposure. The fluoride carrier can range from water tocompounds that will dissolve fluoride but not char upon irradiation. Thefluoride carrier is preferably an inorganic carrier which will not charunder UV irradiation, where the carrier is at least about 70%transparent to UV radiation. This radiation is applied at an energyfluence less than that which will cause ablation of the tooth material.Further, the total thickness of the fluoride containing layer is lessthan that which would make the transmission of the UV radiation to thetooth surface less than about 70%. A suitable thickness is typicallyless than about 1-2 mm. Fluorapatite production on the tooth surfacewill be enhanced without total energy absorption sufficient to increasethe tooth temperature to an amount which would cause pain or damage.

It is also possible to provide both ultraviolet lasers and lasers whichproduce infrared radiation or visible radiation for operation on softtissue. As an alternative, a single laser which is capable of generatingboth infrared and ultraviolet light can be used. Such a laser may be,for example, a frequency multiplied solid state infrared laser such asNd, Ho, or Er:YAG, or a Ti:sapphire laser, or diode lasers. Since it isdifficult to provide lenses which transmit well in both the UV and IRwavelength ranges, the alternative structure, i.e., the use of a curvedconcave mirror in place of the plane mirror 16 and lens 18 in FIG. 1, ispreferred when combined UV and IR wavelengths are used. Interspersedpulses of IR radiation can also be used for sterilization purposes.

In order to prevent the excess build-up of water that is used forcooling during drilling, it is common to remove by suction cariousmaterial as well as blood, tissue, saliva, and water spray. Since thedentist's drill may be subject to contamination, the drill has to beregularly sterilized. In contrast with this, the application of laserpulses is a non-contact technique in which the ultraviolet radiationitself an be used to sterilize the mouth during dental treatment. The UVlight is reduced in intensity below the ablation threshold for thispurpose. This type of ultraviolet treatment can be undertaken before orafter the ablation procedure or, as an alternative, lower power UVpulses can be interspersed among the ablation pulses to neutralize theplume that develops during ablation. An easy way to do this is to use abeam splitter to split the ablation pulses into two pulses where one ofthe pulse is delayed. If desired, the delayed pulse can also be reducedin intensity. The delayed pulse is used to minimize or remove thebioactivity of the plume.

The plume consisting of particulates blown off from the ablation site,including any other matter, can be evacuated by suction during theultraviolet ablation process. FIG. 7 shows one possible technique fordoing this in which a tool 36 held by the dentist contains both anoptical fiber 38 for delivering the ultraviolet radiation to the tooth40, as well as a small tube 42 which acts as the inlet end for a suctionmechanism connected to the vacuum pump 44, and then to a filter (notshown).

As an alternative, the suction tube 42 can be anular tube that iscoaxial with the optical fiber 38, as shown in FIG. 8. Both the fiber 38and the anular suction tube 42 are located in the dental tool 36.

The pulse width and pulse repetition rate of the ultraviolet laserpulses are chosen so that the dominant mechanism for removal of materialis ablative photodecomposition in which there is minimal heat diffusionto surrounding areas of the teeth or gums. While ablation threshold mustbe met or exceeded in order to have ablative photodecomposition, theupper limit on the energy fluence of the pulses is that which wouldcause excessive heat or damage, i.e., energy beyond that which isdesired for etching. Further, the pulse repetition rate and the width ofthe optical pulses are also chosen with the parameters in mind. Excimerlasers are presently available which provide repetition rates of about1-2000 Hz where the typical pulse duration is about 10 nanoseconds.Pulse broadening to about 50-100 ns can be used to minimize fiberdamage. Excess thermal diffusion (which can cause pain and/or charring)is prevented if the pulse width is less than about 100 nm for repetitionrates less than about 20 Hz. For larger pulse widths and/or higherrepetition rates, water cooling can be used to reduce undesirablethermal effects.

In addition to the excimer lasers previously described, nitrogen lasersare available which provide ultraviolet light at 337 nm; however, theselasers are usually of very low power. Also, fluorine lasers areavailable providing outputs at 157 nm. Various commercial lasersoperating at 193 nm are available having pulse repetition rates of 200pules per second where the pulse width is about 15 nanoseconds.Corrective lens elements can be used to provide rounded-square spotsizes of 0.5 mm by 0.5 mm, or less.

FIG. 9 schematically illustrates a laser dental apparatus which uses a"signature" of the ablated material (enamel, dentin, or cariousmaterial) in order to automatically adjust the power output of the laserwhich produces the UV pulses for ablation of the tooth material. In thismanner, the dentist does not have to rely only on his/her expertise indetermining the type of material which is ablated. This can provide avery sensitive control of the energy fluence/pulse, pulse repetitionrate, etc. of the laser output so as to minimize the removal of materialthat is not to be ablated.

In more detail, a laser 46 (excimer, frequency multiplied solid statelaser, etc.) provides ultraviolet pulses which are coupled into adelivery system such as an optical fiber 48 for delivery to an area 50of the tooth 52. These UV laser pulses will have an energy fluence/pulsesufficient to provide ablation of the tooth material being irradiated,where the material can be either enamel, dentin, or carious material. Asnoted previously, ablation of these different materials occurs atdifferent energy thresholds, ablation of these different signatures.Both the "popping sound" and the orange-colored jet emanating from theablated surface are different when the surface being ablated is enamel,dentin, or carious material. For example, the orange-colored jet willstrengthen and/or change color when dentin is ablated, in contrast towhen enamel is ablated (at the same energy fluence). When decayedmaterial is being ablated, the ablation will occur at a significantlygreater rate than the ablation of dentin, which in turn will provide amore pronounced popping sound and a more pronounced orange-colored jet.A sensor which detects either or both this popping sound and theorange-colored jet is used to provide a control signal to the laser 46to control the properties of its output pulses.

In FIG. 9, a second laser 54 provides output radiation pulses which passthrough dichroic mirror 56 and enter optical fiber 58 for delivery tothe region of the plume emanating from the ablated area 50. Dependingupon the material being ablated, different strengths of the orange colorwill appear, providing different wavelengths and/or intensities backinto fiber 58. This return signal reflects from mirror 56 to adetector/analyzer 60. Depending upon the color signature of the plume, asignal is provided to the laser output control unit 62. Control unit 62provides a signal to laser 46 in order to adjust its output power,repetition rate, etc. in accordance with the type of material to beablated.

The output pulses from laser 54 can be delayed with respect to the pulesfrom laser 46, by using a delay unit 64 between the control unit 62 andthe laser 54. In this manner, the output from ablation laser 46 can bequickly adjusted as soon as the detector/analyzer 60 notices that adifferent material is being ablated from region 50 of the tooth 52.Delay unit 64 can be omitted if the system is designed to use the firstablation pulses for analysis of the material being ablated. Afteranalysis, the control unit 62 would adjust laser 46 to set the properenergy of the UV laser pulses.

To provide control using the signature popping sound, a small acousticsensor 49, or fiber optical pressure sensor or fiber optic microphone islocated at the end of fiber 58, to allow it to be close to tooth 52.Sensor 49 is a transducer which provides an electrical signal that issent to the analyzer (60) along line 51. The rest of the feedbackcontrol is the same as that described hereinabove.

FIG. 10 schematically illustrates a laser dental apparatus which can beused to protect against the occurrence of biological contamination, suchas hepatitis, HIV, etc. In this apparatus, a fiber optic biosensor isused to provide recognition of biological conditions in the patient'smouth. For example, if the fiber optic sensor detects the presence of aselected virus in the material being ablated from the tooth, a feedbacksignal can automatically stop the ablation laser, or trigger theablation laser to operate at a lower power in order to provide agermicidal effect in which the unwanted virus is destroyed or minimized,while not causing further ablation of the tooth (and therefore stoppingthe production of additional airborne biological contaminants). Thiscontrasts with the problem of aspirating contaminated material frompatients when conventional mechanical drills are used.

In FIG. 10, a laser 66 provides UV output pulses that are coupled intooptical fiber 68 for delivery to a region 70 of the tooth 72 to beirradiated. Another optical fiber 74 transmits a response from the areaof th irradiated section 70 to a detector 76. The output of the detectoris sent to an analyzer 78 which determines what, if any, harmfulbiological material is present in the plume emanating from theirradiated region 70 when it is ablated. The analyzer 78 then provides asignal to the laser control unit 80, which in turn provides a signal tolaser 66 in order to either turn off laser 66, or to reduce theenergy/pulse of the UV pulses from this laser, or in some other way toalert the operator (for example, an alarm or indicator light).

A layer 82 of a biologically sensitive recognition element (such as anantibody, DNA or a chemical) is located at the distal ends of fibers 68and 74. Dependent on the signal received from the area of the irradiatedregion 70, layer 82 provides a biosensor function, i.e., it recognizesbiological material in the vicinity of the irradiated region. Thismaterial causes a change in the light reflected to the detector alongfiber 74. Different types of fiber optical sensors, including biologicalrecognition elements, are described in an article by David R. Walt,appearing in the Proceedings of the IEEE, Volume 60, No. 6, June 1992,at page 903. The indicating layer 82 modulates light sent down the fiberfrom the laser 66. The amount of modulation of the return light from thearea surrounding the irradiated region 70 can be a measure of the amount(including presence/absence) of a particular material in contact withthe fiber tip.

The sensor layer 82 can be used to provide an indication of temperatureand/or pressure if it is desired to control the amount of heat build-updue to the rate of ablation from the tooth 72. In operation, if thetemperature or pressure goes above a preselected amount, this wouldtrigger a feedback signal to reduce the repetition rate of the laserpulses and/or the energy/pulse. Further, the fiber 74 can be used tocouple radiation of different wavelengths back to the detector 76. Thewavelength being detected will depend on how the system is designed,i.e., what type of signature the system is attempting to locate anddetect.

While separate fibers 68 and 74 have been illustrated, it will beappreciated that a single fiber or fiber bundles can be used. Also abifurcated fiber can be used in which the fiber is split so that theexcitation radiation is transmitted through one portion while the returnsignal is collected through another portion.

The detector 76 is designed to be sensitive to the signature of thespecies being detected. It can be chosen to be responsive to wavelength,intensity, etc. Suitable detectors include various diodes and diodearrays, charge coupled devices, photomultiplier tubes, etc.

U.S. Pat. No. 5,107,516 described a spectroscopic feedback systemutilizing two lasers wherein resonance fluorescence radiation is used toprovide a feedback signal to control the ablation laser. In the systemof that patent, the distinction to be detected is that between arterialplaque and normal tissue. Additional references generally describingfeedback systems for use with lasers in medical applications anddentistry include D. R. Wyman et al., Proceedings of the IEEE, Vol. 80,No. 6, p. 890, June 1992 and German patent DE 40 15066 A1. In contractwith these references, the present invention uses the signatures ofsound and/or color jet and/or tissue temperature described herein tocontrol the ablation laser output used for the treatment of dentaltissues.

While the invention has been described with respect to particularembodiments, it will be appreciated by those of skill in the art thatvariations can be made without departing from the spirit of the presentinvention. It has been taught that there are ablation threshold windowsallowing the use of ultraviolet light to do selective hard tissue andsoft tissue dental procedures, a feature which has not heretofore beenavailable to dentists. Depending upon the ultraviolet wavelength chosen,slightly different ablation thresholds will exist for the various typesof tooth material including enamel, dentin, and carious decay. However,regardless of the wavelength chosen, these ablation thresholds can bedetermined by the procedures described herein and illustrated withresect to the plots of FIGS. 3-6.

The windows of operation described herein make it possible toselectively do other procedures safely, as for example curing resins forbonding to tooth structures, sterilizing root canals and other surfacesand removing tartar from tooth surfaces.

We claim:
 1. An apparatus for performing laser dentistry, comprising incombination:a first laser producing ultraviolet radiation, means forestablishing the energy fluence of said ultraviolet radiation of saidfist laser at an amount sufficient to ablate tooth material, means forestablishing the energy fluence of said ultraviolet radiation at anamount less than that which is sufficient to ablate enamel but greaterthan the threshold amount necessary to ablate carious material, meansfor sending a beam of said ultraviolet radiation to a selected region ofsaid tooth to selectively ablate material therefrom and generate a plumeadjacent to said selected region, and a second laser producing outputradiation and including means for directing the radiation at the plume.2. The apparatus of claim 1, including means for establishing an energyfluence less than the threshold amount necessary to ablate cariousmaterial from said tooth.
 3. The apparatus of claim 1, wherein saidfirst and second laser produce pulsed radiation.
 4. The apparatus ofclaim 3, wherein said means for sending includes an optical fiber fordelivering said radiation to said tooth.
 5. The apparatus of claim 4,further including means for adjusting the repetition rate of said pulsedradiation to be less than that which would cause excessive thermalheating in said tooth.
 6. The apparatus of claim 1, wherein said firstlaser produces radiation in the wavelength range of about 300-400 nm. 7.The apparatus of claim 1, wherein said ultraviolet radiation has awavelength of less than about 220 nm.
 8. The apparatus of claim 1,wherein said first laser produces pulsed radiation in the wavelengthrange of about 100-400 nm.
 9. The apparatus of claim 8, furtherincluding means for adjusting the repetition rate of said laser pulses.10. The apparatus of claim 1, wherein said means for establishing theenergy fluence of said radiation can establish the energy fluence at alevel in excess of about 5.8 J/cm² and also less than about 1.3 J/cm².11. A method for performing laser dentistry, comprising incombination:producing ultraviolet radiation by a first laser,establishing the energy fluence of said ultraviolet radiation of saidfirst laser at an amount sufficient to ablate tooth material,establishing the energy fluence of said ultraviolet radiation at anamount less than that which is sufficient to ablate enamel but greaterthan the threshold amount necessary to ablate carious material, sendinga beam of said ultraviolet radiation to a selected region of said toothto selectively ablate material therefrom and generate a plume adjacentto said selected region, and producing output radiation by a secondlaser and including means for directing the radiation at the plume.